• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

DHT and resistors in parallel w/ heater

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,

I am designing my amp based on DHT (i.e. driver=300B and power=GM70). I have been trying to find maths behind the values of resistors which are connected in parallel (?) with heater/cathode. I guess I know their functio, but what resistance should they be ? Any reference appreciated.

TIA

Yag
 
Do you mean parallel variable resistors? This is often a hum balance resistor.

If you mean series resistor, this will be the cathode resistor. I would seriously advise studying up a bit on how tube circuits work before embarking on such an ambitious project... Perhaps build a 300B amp first, then a GM70?
 
Guys,

Thank you for your care and warnings. I know HV is dangerous as I worked with Roentgen tubes (something like 70kV - 120 kV) and this is not my first tube amp. If I sounded like a newbie, forgive me. I was burning the midnight oil. But ... I have searched web pages for the criteria of choosing such values for the resistors and found none.

To clarify my question - those are two kinds of resistors:
- in series with a heater which limit the filament current,
- in parallel with a heater which balance hum.

If those values are taken arbitrary - what should they be like ? Any experience ? Do they introduce NFB ?
 
analog_sa said:
Can you refer to a particular schematic? Very likely the resistors you mention are also part of the biasing network.

The schematics is attached. I am looking for directions on choosing resisitors:
- Rs1 and Rs2
- Rp1 and Rp2

Simply, I don't to re-invent wheel ;)
 

Attachments

  • res-heater-circ.jpg
    res-heater-circ.jpg
    7.4 KB · Views: 379
i'm not too sure about the function of Rs1 and Rs2.

but for Rp1 and Rp2, they are not required if you are using P1. P1 would be the hum pot. a 100R P1 should be fine. do note wattage has to be sufficient for the current flowing thru.

an alternative is to use just Rp1 and Rp2 without P1. johannes lebong uses 33R for these.

An externally hosted image should be here but it was not working when we last tested it.
 
Hi Yag

Much clearer now. None of these resistors are commonly used. Rs have the effect of reducing the filament voltage, ie if the transformer has higher output than the tube requires. Rp have no particular purpose as far as i can see and may be there to just trim the value of the pot. All these resistors will contribute to the value of Rk and thus have effect on the biasing.
 
Hi analog_sa,

As far as Rs's are concerned, don't they balance heater current either ? What about Rps ? 10x greater a value than heater resistance ? 5x ?
What is the difference if Rp=33 or 22 ohm. The bigger, the better ?

I know you can use a pot instead if its admission power is not exceeded. Yet, I don't see how one takes the values of those. I have seen a lot of schematics of commonly used DHTs, but if I want to use a rare tube, questions arises. Seems like everybody takes the commonly used values, but not one knows why they are such. Looks like dogma ... :(

Audiousername,

I think you are right about the feedback, but I am not quite sure if the extent of degenaration may always be neglected - that is the point I want to get explained. :)
 
Administrator
Joined 2004
Paid Member
The pot would be useful only in the instance where you use AC for heating the output tubes, in practice I have found this does not work well. (I use center tapped transformers in such cases, and only with 45/2A3 at 2.5V) With 300B I typically use 22 - 27.4 ohm 1% fixed resistors, do note that about 100mA of filament current will flow through them and each resistor will have to dissipate about .25W so I would recommend a 1W resistor. The thevenin equivalent series resistance of these resistors is 11 - 13.7 ohms so the current feedback is relatively low, and raises the effective RP by less than 60 ohms worst case.

Dc filament heating is probably going to be necessary for noise considerations. Constant current operation with a CCS works really well and only requires something like an LT1084 with 1.2 ohm resistor between out and reference terminal to set, you take output from reference terminal side. Input voltage should be at least 8V for good performacne despite being LDO.. Also you can use cheap LM317 but need higher input voltage > 9V for good performance. All this presupposes relatively good filtering ahead of the regulator.. Alternately you can use another voltage regulator ahead of the CCS, again allow for drop voltages, this regulator should put out > 8V, and raw dc > 12V . This is similar to the ronan regulator, but simpler and performs as well.

What are you going to use to drive the 300B? I assume you plan to have some gain ahead of it.. Mu is <4 and requires about 90Vpk to drive to full output.. Are you planning to use an interstage transformer and if so what ratio?
 
kevinkr said:


What are you going to use to drive the 300B? I assume you plan to have some gain ahead of it.. Mu is <4 and requires about 90Vpk to drive to full output.. Are you planning to use an interstage transformer and if so what ratio?

Thanks Kevin,

Fist I wanted to use 6SN7 or 6SL7, actually Russian equivalents, but finally I decided to use 6C45 (6S45P) for driving 300B choke loaded (mu=3.8) driving GM70 choke loaded. We shall see. I want to use hybrid Graetz bridge for output stage - Russian 5U8C (lat. 5C8S) and EZ81 for driver and input.

No I don't want use xformes. I would rather remove them if I could. :smash:

Wish me luck. Next weekend I will start.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.